Mammography is the modality of choice for screening for early breast cancer. In mammography, preferably low energy X-rays are employed for examining an object, e.g. human breast tissue, as a diagnostic and screening tool. The goal of mammography is the early detection of breast cancer, typically through detection of characteristic masses and/or micro-calcifications.
In this regard, determining breast density is an important indicator for a cancer risk. With the large amount of images generated in mammography screening programs, it is beneficial to have a reliable automatic breast density assessment (BDA) in order to support the user, e.g. a radiologist.
FIG. 1 shows an exemplary embodiment of a mammography system. Imaging system 2 comprises X-ray source 4 as well as an X-ray detector 6. X-ray detector 6 also doubles as a second compression element 8b, while a first compression element 8a, e.g. a compression paddle, is arranged between X-ray source 4 and X-ray detector 6. The first and second compression element 8a,b are movable relative to one another, in particular, the first compression element 8a is movable relative to the second compression element 8b/X-ray detector 6.
X-ray source 4 is exemplarily rotatable about an axis for obtaining horizontal/parallel as well as oblique image information. In standard mammography, the surface of the detector 6 is regularly always orthogonal to the line connecting X-ray detector 6 and X-ray source 4. For oblique mammographic views, the entire system 4,6,8 is rotatable, as indicated by the circular arrow in FIG. 1. Angulations of X-ray source 4 with respect to a fixed position of the X-ray detector 6 may be used for tomosynthesis applications.
FIG. 2 shows an exemplary mammography screening. An object 10, e.g. human breast tissue, is arranged between the first and second compression elements 8a,b, in particular compressed between the first and second compression elements 8a,b. 
The first compression element 8a is attached to a movable arm element 18 allowing a relative movement of the first compression element 8a relative to the second compression element 8b/X-ray detector 6. By moving the first compression element 8a with force F acting in an object distal region 22, object 10 is compressed between the first and second compression element 8a,b. Object 10 is generating a counterforce F′ in an object proximal region 20 of the first compression element 8a. Since forces F, F′ do not coincide, resulting forces are acting on the first compression element 8a. In particular, a tilt about tilt axis 14 is conceivable as well as, in case the first compression element 8a is not made of an infinite rigid material, a deflection 16 by bending the first compression element 8a. Accordingly, a known distance x between the first and the second compression element 8a,b at the object distal side 22 results in a distance x+Δx at the object proximal side 20 due to counterforce F′.
For determining breast density, precise information about the force F applied to the first compression element 8a as well as the distance x between the first and the second compression element 8a,b is required. Thus, a precise measurement of breast thickness, i.e. distance x, further taking into account additional tilt and deflection of a compression element resulting in Δx is of importance when automatically determining volumetric or mammographic breast density, from image information of digital mammograms. This procedure is also referred to as breast density assessment (BDA). Due to tilt and deflection of a compression element, the actual distance x+Δx may show deviations of Δx=5 mm from the true thickness up to even Δx=15 mm. However, even small errors of 1 to 2 mm may be considered to have a large impact on the breast density assessment, resulting in a significant misjudgement of the density.